This invention relates generally to mode canceling actuators used for canceling vibrational modes experienced by actuator-controlled arm assemblies employed in disk drives.
Actuator-controlled arm assemblies employed in disk drives experience problems due to mechanical resonances. These resonances, or vibrational modes include the natural modes of the actuator system, e.g., a voice coil motor (VCM), and those of the arm assembly.
In a typical disk drive the arm assembly has a positioner arm equipped with a slider. The arm assembly is mounted about a pivot. The slider carries a transducer or read/write head and is located above the surface of a magnetic disk. The VCM is mounted opposite the slider and causes the arm assembly to rotate about the pivot as required to seek and track data on the disk. Specifically, the arm assembly moves the head to a desired data track in the disk and, once there, maintains the head""s position over the track.
At high speeds and large track densities these seek and track operations are affected by the vibrational modes of the arm assembly and the actuator. Specifically, the vibrational modes limit the control loop gain of the actuator system, reduce the bandwidth of the actuator system, or both. This causes the head to experience excessive settling time after positioning, poor response to disturbances, poor tracking ability, or any combination of these.
General methods of achieving higher bandwidth include the use of high specific stiffness materials such as AlBC or Albumet for making the arms, employment of compound actuators and application of active and passive damping techniques.
The use of dual-stage actuation systems having a primary actuator, e.g., a VCM, for executing large movements and a secondary actuator, e.g., a piezoelectric element (PZT), for fine-tuning and tracking is well-known in the art. The small PZT milliactuator has higher vibrational modes than the VCM due to scaling. Descriptions of such systems are found in scientific and patent literature. Some representative references include Zhi-Min Y., et al. xe2x80x9cController Design Criteria for the Dual-Stage Disk Actuator Systemxe2x80x9d, Proc. SPIExe2x80x94International Society for Optical Engineering (USA), Vol. 2101, No. 1, 1993, pp. 305-8 and Guo, W., et al., xe2x80x9cDual Stage Actuators for High Density Rotating Memory Devicesxe2x80x9d, IEEE Trans. Magn. (USA), Vol .34, No. 2, pt. 1 March 1998, pp. 450-5.
Prior art systems have also attempted to ensure stable operation of actuator systems by stabilizing the control loop. This has been done by inserting gain stabilizing filters such as electronic notch filters in the control loop path. These filters are placed in the downstream portion of the control loop to filter out the signal information within the band reject frequency range of the notch and thus help minimize excitation of these vibrational modes.
Filtering techniques of this type have several drawbacks. First, rather than compensating the vibrations of the actuated arm assembly directly they rely on correcting the driving signals sent to the actuator to achieve compensation. Second, these techniques depend on the ability of the designer to accurately predict the frequency of the vibrational modes. This becomes increasingly more difficult when operating in high accuracy regimes. Third, compensation of the gain loop is not capable of eliminating the amplitude and phase effects of the vibrational mode, i.e., the effects of the mode can not be completely canceled.
Another technique for damping vibrational modes of a servo control system was presented by Masahito Kobayashi et al. in xe2x80x9cMR-46 Carriage Acceleration Feedback Multi-Sensing Controller for Sector Servo Systems,xe2x80x9d at the International Conference on Micromechtronics for Information and Precision Equipment, Tokyo, Jul. 20-23, 1997. This proposed multi-sensing control system uses accelerometers to generate acceleration feedback. An acceleration feedback controller receives the feedback signals and compensates the servo to damp the mechanical resonance modes.
Although Kobayashi""s technique has been demonstrated, it can not be efficiently implemented without the use of notch filters. Furthermore, designing the feedback controller requires the designer to model the very complicated transfer function Hd(s) of the servo-controlled system. This is computationally challenging and requires a considerable amount of processing time. Also, since the poles and zeros of the compensator used in the feedback controller can not be predetermined, it is not possible to guarantee the existence of a stable compensator. Most importantly, however, Kobayashi""s technique does not achieve cancellation of the vibrational mode.
The prior art also teaches gain stabilization through low-pass filtering in the control loop. According to this method the components of the control signal having the resonance frequency are effectively prevented from exciting the vibrational modes of the actuator structure. This helps ensure system stability, but it also increases the phase shift at frequencies in the vicinity of the servo loop""s unity gain crossing, thereby reducing the bandwidth of the servo system.
In U.S. Pat. No. 5,459,383 Sidman et al. teach a feedback loop using a motion sensor located in the servo system at or near the point of control. The sensor is referred to as xe2x80x9ccollocatedxe2x80x9d because it is at or near the point of control. During operation the sensor detects both the rigid body motion and deformation of the actuator. The signal component from the rigid body motion is always much larger than that due to deformation. The xe2x80x9ccollocatedxe2x80x9d feedback loop operates in conjunction with the ordinary feedback loop and has the effect of making the servo system perform as if the mechanical structure of the system had a much higher mechanical damping than it actually possesses.
Although Sidman""s system does improve positioning control it relies on gain compensation and does not actually cancel any vibrational modes. Furthermore, the signal derived from the sensor includes the large rigid body component, which is also processed by the feedback loop and affects undesirably the rigid body motion properties of the actuator.
The above-mentioned problems have prevented the development of a servo system capable of entirely eliminating the effects of vibrational modes, including modes at the lowest frequencies which severely limit the bandwidth of the servo system. Moreover, the solutions relying on control signal compensation or passive damping introduce considerable complications into the control loop.
A high-performance disk drive should have, in addition to the ability to cancel at least one vibrational mode, e.g., the butterfly mode, a low-mass arm assembly and its rigid body motion should remain unaffected.
Accordingly, it is a primary object of the present invention to provide a vibrational mode canceling mechanism for stabilizing a servo-controlled actuator system and overcome the disadvantages of the prior art. Specifically, the control mechanism of the invention is designed to cancel the amplitude and phase effects of at least one vibrational mode while minimizing the mass of the arm assembly and preserving its rigid body motion characteristics.
It is another object of the invention to provide for mode cancellation of major modes including the butterfly mode. Still another object of the invention is to achieve mode cancellation with a low-cost and low-mass element, and to thus permit one to design efficient devices with a higher number of tracks per inch (TPI). In particular, it is an object of the invention to employ a mode-canceling actuator such as a piezoelectric (PZT) element.
The above objects and advantages, as well as numerous improvements attained by the system and method of the invention are pointed out below.
These objects and advantages are attained in a disk drive in which an arm assembly is equipped with a primary actuator and a mode-canceling actuator. The primary actuator, preferably a voice coil motor (VCM), is mounted on the assembly at a first location while the mode-canceling actuator, preferably a piezoelectric (PZT) actuator, is mounted at a second location. The second location is selected in such a way that the strain in the mode-canceling actuator is in phase with sway deformations of the primary actuator produced by the vibrational modes. Specifically, the strain experienced by the mode-canceling actuator has to be in phase with sway deformations in the plane of the arm assembly caused by the major modes. A primary driver is provided to control the primary actuator such that it exerts a primary force,             f      ⋁        pm    ,
on the arm assembly. Meanwhile, a secondary driver ensures that the mode-canceling actuator applies a mode-canceling force,             f      ⋁              m      ⁢              xe2x80x83            ⁢      c        ,
to the arm assembly such that one or more vibrational modes, e.g., the lowest frequency modes, are canceled.
The primary and secondary drives are conveniently controlled by a control circuit or device which sets the ratio r of the primary force,             f      ⋁        pm    ,
to the mode-canceling force,       f    ⋁        m    ⁢          xe2x80x83        ⁢    c  
at a certain value. Specifically, the value of ratio r and the location of the mode-canceling actuator are set such that none of the vibrational modes produced by the mode-canceling force exceeds any of the corresponding vibrational modes caused by the primary actuator by more than 6 dB.
Preferably, the mode-canceling actuator eliminates at least the butterfly mode. In some disk drives, elimination of one or more major modes other than the Butterfly mode is also desirable.